Typical cable television communications systems include four main elements, a headend, a trunk system, a distribution system consisting of feeder cables bridged from the trunk system, and subscriber drops fed from broadband signal taps in the distribution system.
The headend, which is the central originating point of all signals carried on the network, receives signals that are broadcasted, transmitted by cable, or transmitted by satellites and transmits these signals as a broadband signal to numerous distribution nodes via trunk cables. Each distribution node, in turn, distributes the signals along with power, via feeder cables, with each feeder cable terminating at a termination block. Each feeder cable has numerous amplifiers and signal taps inserted between the distribution node and the termination block. Typically, an amplifier is provided every half mile, either on a utility pole or in a pedestal on the ground. Each amplifier provides amplification of the broadband signal and each broadband signal tap draws a portion of the amplified broadband signal and power for use by a subscriber.
There are typically three types of signals in a cable television communications system: a "power signal", a "forward signal" from the headend to the subscriber, and a "reverse signal" from the subscriber to the headend. Rather than rely on power from a subscriber circuit, CATV operators typically prefer to supply a power signal that provides electrical power through the same coaxial cable that provides the broadband signal. The forward signal provides the broadband signal containing the audio and video signals for the various cable television channels. The reverse signal is typically a digital signal that allows the subscriber to communicate with the headend, typically to order services in connection with interactive television services. The forward and reverse signals are amplified by the amplifiers at various locations along the feeder cables.
Within each amplifier, there are typically a number of RF directional couplers that operate to split the RF energy in the forward and reverse signals into two portions. Within each coupler, a small portion of the signal is extracted from the input signal and the remaining large portion of the signal is passed through the coupler. The extracted portion of the signal is typically provided to a test point, which allows a service technician to inspect signal levels without interrupting the main signal.
Preferably, each RF coupler should provide impedance matching and high return loss. A prior RF coupler is shown in FIGS. 1A and 1B. The coupler, generally shown at 10, includes a core 12 having two holes 14 therein. The core 12 is typically a ferrite core and is called a dual hole core, or a "binocular" core, because of its shape. Wire windings 16 enter a hole from one side of the core and exit the hole from the opposite side of the core, which requires that the winding return to the hole from outside the core to begin another turn through the hole. Therefore, wire windings pass through the holes and then return to the same hole by looping around the outside of the core.
In FIGS. 1A and 1B, it is seen how the wire windings on the outside of the core are spread out around the periphery of the core, rather than staying closely bunched together. The spacing of the windings outside the core may be due to the lack of a confined path for the windings outside the core or to the need to manually tune the coupler, in which service personnel adjust the spacing of the windings outside the core until a desired return loss and impedance match is obtained. Having the windings located outside the core and spread out outside the core is disadvantageous because it decreases the magnetic coupling of the windings and therefore decreases the performance of the coupler.
Accordingly, there is a need for an RF coupler having high return loss and excellent impedance matching that does not have windings that exit a hole in the core and return to the hole from outside the core.